Transistors are the building blocks of most electronic circuits, such as amplifiers, filters, oscillators, logic gates, switching circuitry, memory devices, programmable logic arrays, and gate arrays to name a few. Transistors are employed in the simplest amplifier or oscillator to the most elaborate digital computer. Integrated circuits are merely an array of transistors and other components built from a single chip of semiconductor material. These electronic circuits are employed in many electronic systems, such as in transmitters and receivers for all types of communication and radar systems.
The transistor is a device that can amplify an input signal to produce a higher power output signal. Transistors come in a variety of different transistor types such as bipolar junction transistors (BJTs), junction field-effect transistors (JFETs), metal-oxide-semiconductor field-effect transistors (MOSFETS), heterojunction bipolar transistors (HBTs) and high electron mobility transistors (HEMTs). Each of these transistor types have unique operating characteristics, and can be selected for a particular application based on these operating characteristics. Each of these transistor types are modeled in a similar manner, such that they have an operating bandwidth that is limited by a cutoff frequency or maximum frequency in which the output gain of the transistor falls below unity.
The desire for systems to provided wider operation bandwidths drive the transistors to operate at higher carrier frequencies, such as microwave frequencies (wavelength less than 1 micrometer) or millimeterwave frequencies (wavelength less than 1 millimeter). However, since the cutoff frequency is related to the intrinsic capacitance that is dictated by the physical periphery of the transistor, common practice is to reduce the size of transistors to increase its cutoff frequency and maximize operating frequency. However, the reduction in size of the transistor limits its power output capability. Therefore, to meet the system level power requirement, smaller periphery transistors need to be combined at a cost of increasing circuit footprint and DC power relative to designs that only requires a single larger transistor.